This invention relates in general to electroluminescent devices and in particular to a double heterojunction semiconductor light emitting diode.
Light emitting diodes (LED's) are frequently used for displays and indicators as well as laser sources. In one type of LED, a p-n junction semiconductor is employed. A potential difference is applied across the junction by means of a pair of electrode contacts in contact with the p-type and n-type regions. This causes electrons to be injected across the junction from the n-type region to the p-type region and causes holes to be injected across the junction from the p-type region to the n-type region. In the p-type region, the injected electrons recombine with the holes resulting in light emissions; in the n-type region, the injected holes recombine with electrons resulting in light emission. The wavelength of the light emission depends on the energy generated by the recombination of electrons and holes; which is known as the band gap of the p-n junction semiconductor material.
To enhance the efficiency of light emission, it is known to those skilled in the art to be preferable to cause only one of the two types of carriers, namely electrons or holes, to be injected across the junction, but not both. In improved LEDs, a p-n single heterojunction semiconductor is employed. A heterojunction is formed at the junction between a p-type and a n-type semiconductor materials of different band gaps. In such heterojunction devices, the energy band gap in the p-type region is different from that in the n-type region so that either electrons or holes, but not both, are injected across the junction. The injected electrons or holes then recombine to cause light emission. Typically, the materials of the n-type and p-type regions are chosen so that only electrons are injected across the junction from the n-type to the p-type region so that the p-type region becomes the only active light emitting portion of the LED.
In conventional LED devices, p-type and n-type layers are grown on a substrate to form the p-n junction. One common substrate material used in LED's is gallium arsenide. If the semiconductor is composed of gallium arsenide (GaAs), the energy band gap of the semiconductor material can be increased with substitution of aluminum atoms for gallium atoms. The greater the content of aluminum in the material, the higher is the band gap. In conventional single heterojunction LEDs employing GaAs as the semiconductor material, a single p-n junction is formed by growing on a GaAs substrate a p-type layer followed by a n-type layer. It is known to substitute aluminum for gallium in both the n-type and p-type regions, where the n-type region contains more aluminum and thus wider band gap than the p-type region. This has the effect of causing the electrons injected from the n-type region to the p-type region to have a lower potential barrier than the holes injected in the opposite direction. Thus, essentially only electrons will be injected across the junction, and the p-type layer is the active layer where radiative recombinations take place.
Since the GaAs substrate on the bottom side and the metal electrode contact at the top absorb the light emitted by the active gallium aluminum arsenide (AlGaAs) p-type region, it is desirable to provide windows between such light absorbent obstacles and the active region to increase the percentage of light that can be utilized. The n-type layer is chosen to be thick to provide a window.
A device known as double heterojunction LED improves on the efficiency of single heterojunction LED's. By inserting an additional p-type layer of higher band gap material between the p-active layer and the substrate, a second junction is effected between the two p-type layers. The extra higher band gap p-type layer helps to confine the injected electrons and prevent them from diffusing deeply into the p-active layer, thereby inducing them to rapidly recombine with the holes to produce more efficient light emissions. Also the extra p-type layer provides a further window for light out of the p-active layer.
In the conventional devices described above, the LEDs have p-type substrates so that the last or top layer being grown is of the n-type. This is disadvantageous for two reasons. Since most of the light will escape from the active layer from the top n-type layer, a small ohmic contact obscuring only a small part of the surface will reduce the amount of blocking. A small contact requires a good bonding to the surface, and sufficiently good contacts to an n-type layer are more complex, such as ones made of gold-germanium alloys. Furthermore, low dislocation p-type GaAs substrates are not as readily available as their n-type counterparts. It is therefore desirable to provide double heterojunction LED structures in which such difficulties are alleviated.